Coastal Lagoons - Chapter 4 docx

4.1.5.1.2 Organic Nitrogen4.1.5.1.3 Ammonium Nitrogen4.1.5.1.4 Nitrate Nitrogen4.1.5.1.5 Organic Nitrogen Benthic4.1.5.1.6 Ammonia Nitrogen Benthic4.1.5.1.7 Nitrate Nitrogen Benthic4.1.5

4.1.1.3 Denitrification4.1.1.4 Nitrate Ammonification4.1.1.5 Mineralization of Organic Nitrogen

(Ammonium Regeneration)4.1.1.6 Ammonia Release from Sediment4.1.1.7 Nitrogen Fixation

4.1.2 Phosphorus Cycle4.1.2.1 Uptake of Phosphorus4.1.2.2 Phytoplankton Death and Mineralization4.1.2.3 Phosphorus Release from Sediment4.1.2.4 Sorption of Phosphorus

4.1.2.5 Significance of N/P Ratio4.1.3 Silicon Cycle

4.1.3.1 Uptake of Silicon4.1.3.2 Settling of Diatoms4.1.3.3 Dissolution of Silica4.1.4 Dissolved Oxygen

4.1.4.1 Processes Affecting the Dissolved Oxygen

Balance in Water4.1.4.1.1 Reaeration4.1.4.1.2 Photosynthesis—Respiration4.1.4.1.3 Oxidation of Organic Matter4.1.4.1.4 Oxidation of Inorganic Matter4.1.4.1.5 Sediment Oxygen Demand4.1.4.1.6 Nitrification

4.1.4.2 Redox Potential4.1.5 Modeling of Nutrient Cycles4.1.5.1 Modeling Nitrogen Cycle

4.1.5.1.1 Phytoplankton Nitrogen4

4.1.5.1.2 Organic Nitrogen4.1.5.1.3 Ammonium Nitrogen4.1.5.1.4 Nitrate Nitrogen4.1.5.1.5 Organic Nitrogen (Benthic)4.1.5.1.6 Ammonia Nitrogen (Benthic)4.1.5.1.7 Nitrate Nitrogen (Benthic)4.1.5.2 Modeling of Phosphorus Cycle

4.1.5.2.1 Inorganic Phosphorus4.1.5.2.2 Phytoplankton Phosphorus4.1.5.2.3 Organic Phosphorus4.1.5.2.4 Organic Phosphorus (Benthic)4.1.5.2.5 Inorganic Phosphorus (Benthic)4.1.5.3 Modeling of Silicon Cycle

4.1.5.4 Modeling of Dissolved Oxygen

4.1.5.4.1 Dissolved Oxygen4.1.5.4.2 Dissolved Oxygen (Benthic)4.1.5.4.3 Sediment Oxygen Demand4.2 Organic Chemicals

4.2.1 Sources of Organic Chemicals4.2.2 Classification of Organic Chemicals That Might Appear

in Aquatic Environments4.2.3 Fate of Organic Chemicals in Aquatic Environments4.2.3.1 Volatilization

4.2.3.2 Ionization4.2.3.3 Sorption4.2.3.4 Hydrolysis4.2.3.5 Oxidation4.2.3.6 Photolysis4.2.3.7 Biodegradation4.2.4 Governing Equations of Reactions To Be Used in Modeling4.2.4.1 Volatilization

4.2.4.2 Sorption4.2.4.3 Computation of Partition Coefficients4.2.4.4 Hydrolysis

4.2.4.5 Oxidation4.2.4.6 Photolysis4.2.4.7 BiodegradationAcknowledgments

References

4.1 NUTRIENT CYCLES

Among the most productive ecosystems in the biosphere, coastal lagoons cover 13%

of world’s coastal zone1 and constitute an interface between terrestrial and marineenvironments.2,3 Nutrient loadings coming from both boundaries to lagoon ecosys-tems have increased considerably in recent years, and they have a major impact on

water quality and ecology.4,5 Control of nutrients is thus one of the major problemsfaced by those responsible for the management of these sensitive ecosystems Inorder to develop appropriate modeling strategies for making scientifically soundapproaches to reduce the risk of environmental degradation of these ecosystems, abetter understanding of nutrient cycles is required.

In this section, nutrient cycles and their associated mechanisms and majorreactions in coastal marine environments are described Additional information oneutrophication caused by nutrient loading will be presented in Chapter 5

4.1.1 N ITROGEN C YCLE

Among nutrients, nitrogen is of particular importance because it is one of the majorfactors regulating primary production in coastal marine environments.6–8 Nutrientsare imported to coastal lagoons via atmosphere, agricultural lands, forests, rivers,urban and suburban run-off, domestic and industrial wastewater discharges, ground-water, and the sea Nutrients are exported via tidal exchange, sediment accumulation,and denitrification An additional source is nitrogen fixation Internal sources ofnitrogen include benthic and pelagic regeneration In general, little is known aboutthe supply of nutrients from the atmosphere and groundwater to coastal lagoons.9

The nitrogen forms that are important in aquatic environments are ammonia/ammonium (NH4+/NH3), nitrate (NO3−), nitrite (NO2−), nitrogen gas (N2), and organicnitrogen These different forms of nitrogen, present in different oxidation states,undergo oxidation and reduction reactions Ammonia and oxidized forms of nitrogen(NO2−, NO3−) constitute dissolved inorganic nitrogen (DIN), which can be utilized

by phytoplankton for growth or by bacteria as an electron acceptor Typical trations of NH4+ and NO3− in coastal waters range from <1–10 µM and <2–25 µM,respectively.10 The various nitrogen compounds and their oxidation states, togetherwith their molecular formulas, are given in Table 4.1

(NH3) The latter form is toxic to aquatic organisms and is in equilibrium with theammonium and hydrogen cations The concentrations of these forms vary consid-erably as a function of pH and temperature in natural water bodies The method ofcalculation of the percent of total ammonia that is unionized at different pH andtemperature is given in Emerson et al.11

(4.1)

TABLE 4.1 Forms of Nitrogen and Their Oxidation States

Forms of Nitrogen Molecular Formula Oxidation State of N

Nitrogen compounds can be classified into organic and inorganic nitrogen Organicnitrogen in water bodies can be found in both dissolved and particulate forms Theparticulate organic nitrogen (PON) is composed of organic detritus particles andphytoplankton and has two possible fates Dead plant cells lyse and bacteria degradethe resulting dissolved organic nitrogen (DON) or protozoa/zooplankton to consumePON.12 Most of the DON in seawater is still chemically uncharacterized, and itschemical and biological properties are becoming better known.7 Except for aminoacids and urea, which comprise only a small fraction of DON, most of the DONmay be resistant to decomposers.10 Excretion by animals also releases dissolvednitrogen Zooplanktons excrete free amino acids, ammonia, and urea Fish excreteammonia, urea, and other organic compounds.7

In aquatic ecosystems, a very complex biogeochemical nitrogen cycle isobserved (Figure 4.1) The following sections give information about the processesinvolved in the biogeochemical cycling of nitrogen in the aquatic environment

4.1.1.1 Uptake of Nitrogen Forms

Primary production in coastal waters is largely regulated by the availability of NH4+and NO3− for growth Ammonium is preferred by phytoplankton, as its oxidation

FIGURE 4.1 Nitrogen cycle.

Zoo- plankton

Phyto-Organic Detritus

state is equivalent to that of cellular nitrogen (−3) and thus requires the least energyfor assimilation.12,13 Ammonia concentrations above 1–2 µM tend to inhibit assim-ilation of other nitrogen species.10 On the other hand, if nitrate is to be assimilatedfor the synthesis of cellular materials, it should be reduced to ammonia with the aid

of several enzymes including nitrate reductase (enzyme catalyzed reduction) withinthe cell This reduction process is called “assimilatory nitrate reduction” and requiresenergy.7,14

Nitrogen uptake can be an important process For example, in Basin d’Arcachon

in southern France, due to the high nitrogen uptake rates of the seagrass Zostera noltii, nitrogen uptake is quantitatively more important than denitrification as a

nitrogen sink.15

In shallow water systems, biological organisms larger than phytoplanktonturn over slowly, and their metabolism is lower Nevertheless, these organismsstore large amounts of nitrogen, because a substantial amount of nitrogen is tied

up in their biology Thus, nitrogen concentrations in the shallow systems tend to

be lower.16

Nutrient assimilation by macrophytes can be significantly different from that byphytoplankton because macrophytes have the ability to grow for long periods onstored nutrients Rooted seagrasses can assimilate nutrients from sediment andpossibly serve as nutrient pumps10 (see Chapter 5 for details)

4.1.1.2 Nitrification

Nitrification is the microbiological oxidation of ammonium to nitrite and then tonitrate under aerobic conditions, to satisfy the energy requirements of autotrophicmicroorganisms Much of the energy released by this oxidation is used to reducethe carbon present in CO2 to the oxidation state of cellular carbon, during theformation of organic matter

As indicated previously, the first step in nitrification is oxidation of ammonium

to nitrite, which is accomplished by Nitrosomonas bacteria.

(4.2)

The second step is oxidation of nitrite to nitrate by Nitrobacter This is a faster

process

(4.3)The overall nitrification reaction is therefore

of NH4+ that is needed by phytoplankton for growth.17 The coupling of the nitrificationprocess with denitrification leads to loss of nitrogen from the atmosphere.

Nitrification can take place either in the water column or in the sediment.However, nitrification in the water column of shallow marine and estuarinesystems appears to be relatively limited.17 Nitrification rates in the water columnare at least in order of magnitude smaller than the nitrification rates per unitvolume in sediment For example, in coastal waters, nitrification rates range fromonly ~0.001–0.1 µmol l− 1 h−1, whereas in coastal sediment nitrification rates areoften 20 µmol l− 1 h−1.18 Nitrification rates measured in coastal sediment are usually

on the order of 30–100 µmol m− 2 h−1.17,10

Physico-chemical and biological factors regulating nitrification in coastal marinesediment include temperature, light, NH4+ concentration, dissolved oxygen concen-tration, pH, dissolved CO2 concentration, salinity, the presence of any inhibitorycompounds, macrofaunal activity, and the presence of macrophyte roots.8,17

Temperature influences the metabolic activities of nitrification bacteria Theoptimum temperature is in the range of 25–35°C in pure cultures.17 Due to bothseasonal and diurnal changes in temperature in shallow coastal sediment, it isexpected that nitrifying bacteria would exhibit optimal growth and/or activity during

of temperature on nitrification rates in pure cultures is usually expressed throughArrhenius type equations.17 In addition, temperature also affects dissolved oxygensolubility and therefore the process rates Light may influence the nitrificationactivity in shallow water sediment Light availability and the penetration depth oflight into sediment may affect benthic nitrification.17

Nitrification may be strongly impeded by hypoxia since it occurs only underaerobic conditions.19 Nitrifying bacteria, therefore, have to compete with otherheterotrophs for the limited supply of dissolved oxygen The depth distribution ofnitrifying bacteria in sediment is ultimately constrained by the downward dissolved

Étang du Prévost in southern France,21 and Danish coastal zones,8 O2 penetrationinto sediment declines due to increased temperature, organic inputs, and decreasedmacrofaunal activity in summer Consequently, thinning of the surficial oxidizedzone of sediment is responsible for the significant summer reduction in nitrificationrates in these systems The reported dissolved oxygen concentrations, which inhibitnitrification in sediment are in the range 1.1–6.2 µM O2.8

Salinity is another factor influencing nitrification Although nitrifying bacteriaare able to acclimate to a wide range of salinities, such as those found in lagoonsystems, short-term fluctuations may have strong regulating effects on nitrifica-

tion For example, a marine Nitrosomonas sp isolated from the Ems-Dollard

estuary at 15% salinity was able to adapt to the entire salinity range (0–35%) andgrew at the same rate over the range after a lag phase of up to 12 days.17 Rysgaard

et al.21 reported in their study conducted with the sediment from the RandersFjord Estuary, Denmark that both nitrifying and denitrifying bacteria were phys-iologically influenced by the presence of sea salt, showing lower activities athigher salinities

Salinity has another, nonphysiological effect on nitrification processes As aconsequence of higher salinity, the concentration of cations also increases Thesecompete with NH4+for adsorption on the sediment As a result, the residence time

increases At higher salinities, NH4+ might diffuse out of the sediment before fication can take place.21

nitri-Rooted benthic macrophytes might also influence nitrification–denitrification cesses in deeper sediment because they release O2 via their roots This release couldstimulate nitrification and thus provide an additional NO3− source for denitrification.15

pro-Sulfide, the product of anaerobic sulfate reduction, is quantitatively the mostimportant toxic sulfur compound in marine sediment.17 Sulfide concentrations cansignificantly reduce the activity of nitrifying bacteria by lowering the redox potential,20

and concentrations between 0.9 and 40 µM can inhibit nitrification completely.18 HS−concentrations in estuarine sediment commonly range from 7–200 µM, which is muchlower than those for organic-rich sediment (>1 mM) The range of HS− concentration

in freshwater sediment pore water is much lower (0–30 µM).22

The presence of nitrifying bacteria in anaerobic sediment at depths well belowthe zone into which oxygen can penetrate is attributable to macrofaunal irrigation

of sediment by physical resuspension and bioturbation. 15,20

4.1.1.3 Denitrification

Denitrification is the microbiological reduction of nitrate to nitrogen gas, wherefacultative heterotrophic organisms use nitrate as the terminal electron acceptorunder anoxic conditions:

(4.5)Nitrogen gas is largely unavailable to support primary production; therefore, deni-trification removes a substantial portion of the biologically available nitrogen andrepresents a mechanism for partial buffering against coastal eutrophication.18,22

The nitrification and denitrification processes taking place in the sediment and

in the sediment–water interface are schematically shown in Figure 4.2 Severalfactors affect denitrification rate, including temperature, pH, redox potential, as well

as concentrations of oxygen, nitrate, and organic matter.7,8,13,14,18

Denitrification rate is highly temperature dependent and generally increases withincreasing temperature.7 However, because of other factors such as nitrification rateand oxygen concentration, which also are temperature dependent it is difficult,especially in sediment, to separate the effect of temperature alone.18

The rate of denitrification decreases with acidity.13,23 The pH range, wheredenitrifiers are most active, is given as 5.8–9.2.7

In marine systems, one of the most important environmental factors favoringdenitrification is the availability of organic matter.14 Simple organic compounds,such as formate, lactate, or glucose, usually serve as the electron donor in addition

to their assimilation Coastal marine environments act as centers of deposition for

NO3−→NO2−→NO→N O2 →N2

continentally derived organic materials Thus, most denitrification in marine ment occurs in coastal regions rather than deep-sea environments.22

sedi-Oxygen concentrations can also affect denitrification rates Denitrification is erally considered to occur only under low oxygen or anaerobic conditions.7 To explaincoupled nitrification–denitrification processes in sediment, it is often assumed thatthese processes are separated vertically within the sediment However, denitrificationcan also occur within reduced microzones in the aerobic surface layer of sediment Inboth freshwater and marine systems, an oxygen concentration of 0.2 mg l−1 or less isrequired for denitrification to occur in the water or sediment.18 Bonin and Raymond24

gen-studied the kinetics of denitrification under different oxygen concentrations using

Pseudomonas nautica isolated from marine sediment They reported that denitrification

can take place in the presence of oxygen However, enzymes associated with fication are affected by the presence of oxygen Nitrate reductase enzyme was com-pletely inhibited at oxygen concentrations greater than 4.05 mg l−1, compared with2.15 mg l−1 and 0.25 mg l−1 for nitrite and nitrous oxide reductase enzymes, respec-tively Yet, these results must not be generalized to all denitrifying strains becausesome bacteria are inhibited by oxygen while other species are not

denitri-In many coastal environments, seasonal trends in denitrification are determinedlargely by availability of NO3− which is controlled by rates of nitrification.20 Theresponse of denitrification rates in sediment slurries to increasing nitrate concentra-tions can often be described by Michaelis-Menten type kinetics The half-saturationconstant for marine sediment generally ranges from 27–53 µM NO3−.18

Supplies of NO3− for denitrification in coastal marine sediment appear to bederived almost exclusively from sediment nitrification.17 Diffusion of nitrate fromthe overlying water into the sediment is also a potential nitrate source for denitrifi-cation, and its rate in the sediment is 3–4 orders of magnitude greater than that ofthe overlying water There is also evidence that the release rates of nitrate andammonium from sediment are greater than their diffusion rates into the sediment.Nitrification is usually observed in the upper 5 cm of sediment, and the nitrateproduced diffuses either up to the water or down to the anoxic zone, where denitri-fication takes place.18,23

FIGURE 4.2 Nitrification and denitrification in sediments.

ammonification burial

ammonification

WATER SEDIMENT

Macrophytes, benthic algae, and certain macrofauna have been shown to ence denitrification rates in both freshwater and marine sediment by affecting theoxygen and/or the nitrate distribution in the sediment.18

influ-A wide range of experimental methodologies has been developed to estimatedenitrification rates in shallow marine environments These techniques are based ondifferent assumptions; therefore, care must be taken when comparing denitrificationrates obtained using these different techniques Seitzinger18 has given the ranges ofdenitrification rates as 50–250 µmol N m− 2 h−1 in estuarine and coastal marinesediment, 2–171 µmol N m− 2 h−1 in lake sediment, and 0–345 µmol N m− 2 h−1 inriver and stream sediment In low oxygen hypolimnetic lake waters, denitrificationrates were generally 0.2–1.9 µmol N l− 1d−1 The higher rates were from systems thatreceive substantial amounts of anthropogenic nutrient input Groundwater is anothernitrate source.18

4.1.1.4 Nitrate Ammonification

Denitrification is widely accepted as the dominant process of nitrate reduction inmost shallow marine sediment An alternate pathway to denitrification is nitrateammonification, which is the reduction of NO3− to NH4+ by heterotrophic bacteria

In contrast to denitrification, nitrogen is not lost from the system but converted to

a readily available nitrogen form.8 Nitrate ammonification can occur occasionallyunder anaerobic conditions.2

Nitrate ammonification is also called dissimilatory nitrate reduction, and it has beendescribed as an important process in marine sediment.24 In both Bassin d’Arcachon andÉtang du Prévost, two coastal lagoons in southern France, rates of nitrate ammonifica-tion were quantitatively as important as denitrification.15

4.1.1.5 Mineralization of Organic Nitrogen (Ammonium Regeneration)

The process of transforming organic compounds back to inorganic compounds

is generally referred to as mineralization.25 Through the mineralization of organicnitrogen compounds, nitrogen recycling is accomplished Recycled nitrogen isprimarily in the form of ammonia and urea (a dissolved organic nitrogen com-pound) Urea is rapidly broken down to ammonia by bacteria or by the extracel-

animal excretion and by microbial decomposition of organic matter It is presumedthat excretion contributes to the largest part of NH4+ regeneration in the watercolumn, while decomposition of organic matter is the most important in thesediment.10

It is widely accepted that shallow coastal sediments are important sites for themineralization of organic matter The difference of shallow coastal waters comparedwith open seas is that a much larger fraction of the organic matter is mineralized

on the bottom rather than in the water column.2,26 Because of the shallow depth ofcoastal areas (e.g., 2–20 m) and the relatively rapid settling rates, a significant portion

of the primary production is transferred to the sediment Thus, much of the alization of nutrients occurs in the upper layer of the sediment.2,18,27,28

miner-Organic compounds are mineralized through both aerobic and anaerobic piration processes Aerobic respiration, which takes place in the surface sedimentlayers (typically 0–5 mm depth), results in a rapid depletion of oxygen In thesediment, bacteria oxidize a significant fraction of the organic matter using terminalelectron acceptors other than oxygen (e.g., nitrate, manganese and iron compounds,sulfate, and carbon dioxide).8,29 The two dominant anaerobic processes are dissim-ilatory sulfate reduction and methanogenesis (methane production) Generally,sulfate reduction precedes methanogenesis because sulfate-reducing bacteria out-compete methanogens for substrates Freshwater has lower sulfate concentrations

reduction occurs deeper in the sediment column (>10 cm) and provides an tional source of NH4+.26 Sulfate reduction, and subsequent inhibition of nitrificationand denitrification by HS−, should lead to enhanced ammonium regeneration duringsummer, when sulfate reduction rates are high compared with those in winter.22 Inall cases, the mineralization of organic nitrogen compounds results in the produc-tion of NH3/NH4+

addi-All living matter contains nitrogenous macromolecules, which become available

to decomposer organisms upon the death of cells Depending upon the structuralcomplexity of the organic matter, mineralization can either be a simple deaminationreaction or a complex series of metabolic steps involving a number of hydrolyticenzymes Thus, mineralization rates depend on the degradability of the organicmatter; i.e., whether it is labile or highly refractory For example, seagrass detritusthat has 25–30% lignin containing fibers, has a lower mineralization rate than

parameter affecting the mineralization rate of organic matter is temperature.7 sonal patterns of benthic nutrient regeneration generally exhibit strong summermaxima, which correlate well with water temperature The effects of temperaturecan be represented by Arrhenius type expressions.27

Sea-Mineralization of organic nitrogen plays a central role in nitrogen recycling incoastal marine environments Regeneration from the sediment regulates all produc-tivity since inorganic nutrients are the limiting factors for primary production,30 andmuch of the primary production of many coastal marine systems is supported bynutrient recycling rather than by nutrient inputs alone.26 In shallow water ecosystems,benthic recycling may account for 20–80% of the nitrogen requirements of thephytoplankton.8,27 Nixon26 reported that nutrient inputs to Narragansett Bay, U.S.A.(without being recycled) could support, at the most, only 24–50% of the annualproduction, depending on the nutrient considered

Ammonium produced during the deamination of organic nitrogen in ment is not totally available to the primary producers; some of the ammoniumremains dissolved in interstitial water, some is adsorbed and buried into deepersediment,7 some is consumed by benthic algae for cell synthesis,13 and a fractionundergoes nitrification in the surficial oxic zone of the sediment.8 Denitrificationfollowing nitrification produces gaseous forms of nitrogen (N2, N2O) essentially

nitrification–denitrification represent a sink that shunts nitrogen away from cling pathways.20

recy-4.1.1.6 Ammonia Release from Sediment

Most of the nitrogen mineralized in the sediment is recycled by diffusion from thesediment to the overlying water as NH4+ or NO3− Some nitrogen can also be released

as urea and dissolved organic nitrogen, but the quantities of these fluxes areunknown.2 There are a few important factors influencing the quantity of NH4+ and

NO3− release, which are explained below

As stated previously, benthic regeneration is a function of temperature NH4+regeneration rates and pore-water concentrations tend to increase with temperature.26

Factors such as sediment grain size and physical circulation also influence thisprocess It has been demonstrated that the activity of (meio- and macro-) faunaenhances the rate of NH4+release from sediment.10

The amounts of NH4+ and NO3− depend greatly on seasonal conditions Forexample, the major part of nitrogen released from the sediment is NH4+ in summerwhen the mineralization rate is high and the aerobic zone depth is generally small

On the other hand, nitrification dominates during winter and spring when the aerobiczone is deeper, and, therefore, NO3− is released from the sediment

In the presence of anaerobic conditions, redox potential decreases significantly,resulting in the termination of nitrification in the sediment The loss of the oxicmicrozone between the sediment and overlying water under anoxic conditions alsocauses a considerable decrease in adsorption capacity of the sediment, producing asignificant increase in the release of NH4+ from the sediment

Salinity is another factor influencing ammonium release from sediment Ambientexchangeable ammonium concentrations in freshwater sediment are generally consid-erably greater than those reported for marine sediment.32 Fluctuating salinity plays amajor role in controlling the NH4+ adsorption capacity of the sediment.21 Specifically,the total amount of cations (primarily Na+, Mg2 +) increases with salinity, resulting ingreater molecular competition with ammonium for the sediment cation exchangesites.21,32,33

The greater ammonium adsorption in freshwater sediment relative to marinesediment increases the amount of ammonium that can be nitrified Seitzinger et al.32

reported in their study that a larger percentage of net ammonium produced in aerobicfreshwater sediment (Toms River, U.S.A.) was nitrified and denitrified (80–100%)compared with that in marine sediment (40–60%) (Barnegat Bay, U.S.A.)

Postma et al.10 reported an NH4+ flux range of 50–800 µmol m− 2 h−1 for estuarineand coastal sediment According to Day et al.27 the annual mean value of nutrientregeneration ranges from 20–300 µmol m− 2 h−1 for NH4+ in estuarine sediment Therate of release of ammonia by a wide variety of marine sediment during summer wasgiven as 13–710 µmol m− 2 h−1 by Nixon.26 NH4+ fluxes in Chesapeake Bay, U.S.A.,were reported to be 46 µmol m− 2 h−1 in April, increasing to 753 µmol m− 2 h−1 inAugust.20 The calculated benthic NH4+ flux at Thau Lagoon, France, for a period of

10 days in August during anoxia was 600 µmol m− 2 h−1.28

4.1.1.7 Nitrogen Fixation

The process that converts atmospheric nitrogen gas (N2) into organic nitrogen pounds is known as nitrogen fixation Most organisms, due to the significant amount

com-of energy required to split NN triple bonds, cannot use the gaseous N2 form A fewgenera of bacteria, blue-green algae (cyanobacteria) that possess heterocytes (special-ized cells present in most filamentous blue-green algae) and some unicellular forms

of blue-green algae without heterocytes are capable of nitrogen fixation Nitrogenfixation may be crucial to the acceleration of eutrophication in aquatic environments,since fixation can occur when other sources of nitrogen are not available or areinsufficient for biological growth.8,13,23 However, in almost all rivers, lakes, and coastalmarine ecosystems, loss of nitrogen via denitrification exceeds the inputs of nitrogenvia N2 fixation.18 For example, in Narragansett Bay, U.S.A., a small amount of nitrogenfixation occurs in the sediment, but it is insignificant compared with nitrogen lossesthrough denitrification (less than 0.0007 mol N m−2 year−1 fixed in sediment comparedwith 0.52 mol N m−2 year−1 denitrified).34

Nitrogen fixation is less effective in making up nitrogen in marine systems than

in freshwater systems as the rates of nitrogen fixation in marine waters are generallylower than those in freshwaters.35 Many hypotheses have been proposed to explainthe difference in the rates of N2 fixation in fresh and marine waters.7,10 The mostlikely explanation is the lower availability of two trace metals (iron and molybde-num) in seawater that are required for nitrogen fixation, compared with their

involved in nitrogen fixation The abundant sulfate in seawater interferes with theuptake of molybdenum by fixers because of its steric similarity to molybdate;therefore, seawater sulfate could reduce the activity of N2 fixers.7 In addition,relatively large amounts of iron are required for the growth of cyanobacteria using

N2 rather than NH4+ or NO3−as the nitrogen source Thus, the rate of N2 fixationmight also be restricted by the low abundance of iron in seawater compared withfreshwater.7

N2 fixation by benthic cyanobacteria can be significant due to the direct supply

of iron and molybdenum from the sediment However, low light penetration canlimit the growth of benthic N2 fixers.34 In unvegetated shallow coastal lagoons andintertidal sediment where light is not limiting, dense populations of benthic nitro-gen-fixing cyanobacteria can locally develop and contribute to nitrogen fixation.However, even though the nitrogen fixed by cyanobacteria is locally important tothe mat communities themselves, the contribution to the total nitrogen budget isminor in most shallow marine ecosystems due to the restricted distributions ofmat communities.8 Since the cyanobacteria population varies greatly in time andspace, detailed measurements are required to estimate the total annual nitrogenfixation rate.13,23

Other physico-chemical parameters influencing nitrogen fixation activity inbenthic sediment include carbon availability, temperature, pH, dissolved oxygen,inorganic nitrogen, and salinity.8

4.1.2 P HOSPHORUS C YCLE

Phosphorus is one of the limiting nutrients for the growth of microorganismsalthough the quantities of phosphorus needed are much smaller than those of C, Si,

or N.23

There are many external sources of phosphorus for coastal ecosystems Domesticwastewater discharges may often contain large quantities of phosphorus becausemany commercial cleaning products contain phosphorus There are also industrialsources of phosphorus, such as wastewater discharge from boiler water treatmentoperations Phosphates applied as fertilizers to agricultural or residential cultivatedland are also transported into surface waters with surface run-off Internal sources

of phosphorus include benthic and pelagic regeneration

Phosphorus in water can be classified into particulate and dissolved forms

Par-ticulate phosphorus includes phosphorus in organisms in/sorbed to dead organic

matter and in/sorbed to mineral phases of rock and soil Dissolved phosphorus iscomposed of orthophosphate (PO4−3) polyphosphates (often originating from syn-thetic detergents), organic colloids, and phosphorus combined with adsorptive col-loids and low-molecular-weight phosphate esters Orthophosphate is the most signif-icant form of phosphorus available for phytoplankton growth Orthophosphate ionsinclude phosphoric acid (H3PO4), its dissociation products (H2PO4−, HPO4−, PO4−),and the ion pairs and complexes of these products with other constituents in seawater.The phosphorus atom has an oxidation state of +5 in orthophosphates.7

General phosphorus transformation mechanisms are illustrated in Figure 4.3 Asseen from this figure, phosphorus can undergo various reactions, depending upon

FIGURE 4.3 Phosphorus cycle.

Nonliving Particulate Organic Phosphorus

Dissolved Organic Phosphorus

death

Rooted Aquatic Plants

Nonliving Particulate Organic Phosphorus

decomposition

hydrolysis

Dissolved Organic Phosphorus

Dissolved Inorganic Phosphorus

Dissolved Inorganic Phosphorus (PO4−

)

mineralization complex formation Phosphorus Complexes (Fe, Mn) adsorption

adsorption

desorption

Particulate Inorganic Phosphorus

Insoluble Phosphorus Compounds

Insoluble Phosphorus Compounds

precipitation setting

WATER SEDIMENT

Particulate Inorganic Phosphorus

desorption mineralization

environmental conditions Orthophosphate is taken up by phytoplankton and rated into cells during growth Some fraction of the phosphorus taken up is released

incorpo-in forms readily available to other algal cells Other phosphorus compounds releasedhave to be mineralized and/or hydrolyzed into inorganic form before their use ingrowth It is likely that phosphorus is also excreted directly by invertebrates, similar

to zooplankton.13 Algal growth produces an increase in particulate organic phosphorus(POP), and the death of algae releases dissolved organic phosphorus (DOP) Some ofthe POP is transformed to dissolved inorganic phosphorus (DIP) as particles decay inthe water column, but some POP settles onto the bottom sediment In the sediment,further degradation of settled organic phosphorus to DIP can take place, and some ofthis formed DIP is subsequently precipitated or adsorbed.7 Bacteria excrete somephosphate, and DIP is also generated by microbial hydrolysis of the esters of the DOP

4.1.2.1 Uptake of Phosphorus

DIP (orthophosphate) is the only form of phosphorus that can be assimilated bybacteria, algae, and plants.13,36 The growth process is usually represented by the

Monod equation, and the half-saturation concentration of nutrients (Ks) in the Monod

equation varies depending on the organisms involved

An organism with a lower Ks value, has the advantage over other organisms

when the nutrient in question (here, phosphorus) is in short supply.27 For example,

algae have lower Ks concentrations (0.6 –1.7 µM) while bacteria have higher Ks

important reservoir than bacteria

Because aquatic macrophytes have the ability to use sedimentary nutrientsources37 and the nutrient concentrations in sediment are much higher than those in

the water column, high Ks values for macrophytes do not result in severe nutrient

limitation.27

4.1.2.2 Phytoplankton Death and Mineralization

During respiration and death of phytoplankton, a fraction of the phosphorus released

is in inorganic form The remaining fraction is in organic form, which must bemineralized and/or hydrolyzed into inorganic form to be made available to otherorganisms This transformation usually occurs in the sediment in coastal ecosystemsbecause these systems are shallow and the settling process is more effective Afterthe transformation, inorganic phosphorus is either released to the overlying waterand made available for growth or adsorbed or buried deeper into the sediment

In fish farms located in lagoon systems, the fish are fed with rich food distributed

in large quantities in the water column This input could constitute an additionalsource of organic matter to the sediment that might increase benthic nutrient fluxes

On the other hand, the greater recycling rates of organic matter could explain thesignificant increase in benthic nutrient fluxes observed in the lagoon fish culturesystems.38

The release of available phosphorus from sediment to overlying water stimulatesprimary production in the water column and is known as regeneration of phosphorus

This can be a significant process in coastal waters Across a variety of coastalsediment, the regeneration of phosphate provides an average of 28% of the phy-toplankton requirements whereas, in Narragansett Bay, U.S.A., it provides 50%.7

Floating and submerged macrophytes also are important phosphorus sourcessince they release phosphorus rapidly (within days) from the decaying leaves androots.13

Phosphorus cycles rapidly through the aquatic food chain, and is seldom limiting

in the marine environment.25 Howarth34 states that during microbial decomposition,phosphorus is released faster than nitrogen, presumably because the ester bonds ofphosphorus are more easily broken than are the covalent bonds of nitrogen

4.1.2.3 Phosphorus Release from Sediment

Phosphorus retention and subsequent release from sediment to overlying waters may

be important in preventing/delaying the improvement of water quality Therefore,much study has been devoted to the phosphorus content of sediment and its move-ment into the overlying water.39

Exchanges across the sediment–water interface are regulated by mechanismsassociated with mineral–water equilibria, sorption processes (notably ion exchange),oxygen-dependent redox reactions and microbial activities, as well as the environmen-tal control of inorganic and organic compounds, i.e., enzymatic reactions The release

of adsorbed phosphorus from sediment is controlled by physical-chemical factors such

as temperature, pH, and redox potential.13 Lower redox potentials and high pH values

in the surface sediment cause phosphorus release during summer, while low atures, high redox potentials, and neutral pH help to retain phosphorus in sediment inwinter.35 The rates of release of phosphate by a wide variety of marine sediment duringsummer are reported by Nixon,26 as −15–50 µmol m− 2 h−1 Release rates are alsoinfluenced by variable rates of turbulent diffusion and the burrowing activities ofbenthic invertebrates.13

temper-The oxygen content of the sediment–water interface is one of the most importantfeatures of the interface As long as a few millimeters of the sediment is aerobic, thephosphorus will be retained in the sediment efficiently Phosphorus binding with ferricoxides is particularly strong At a neutral pH and redox potential greater than 200 mV,Fe(OH)3 is stable,13 and sorption (chemisorption) of orthophosphate takes place Ifthe pore water becomes anaerobic due to respiratory activity in the sediment, ferriciron (Fe+3) is reduced to ferrous iron (Fe2 +) and the binding is weakened.36 This redoxreaction causes the amorphous ferric oxyhydroxides to dissolve, making them unavail-able to adsorb phosphates The dissolved phosphate can leave the anaerobic sediment,but some of the phosphate may precipitate as FePO4 at the oxic-anaerobic interface,and more is probably adsorbed onto the amorphous ferric oxyhydroxides, also at theoxic-anaerobic interphase In any case, the release of phosphate from anaerobicsediment is faster than reoxidation and immobilization, resulting in a net phosphateregeneration from anaerobic sediment into overlying water.7,13

Caraco et al.40 state that there is a correlation between sulfate abundance andphosphorus release from anaerobic sediment that is a result of the interaction betweeniron and sulfur Enhanced sulfate reduction and the resulting formation of iron–sulfide

mineral (e.g., FeS, FeS2) were responsible for increased pore water phosphate mulation However, in sulfate-free sediment, much of the phosphate released duringanaerobic microbial reduction of Fe+3 was captured by solid phase reduced ironcompounds (e.g., Fe3(PO4)2) In freshwater systems, sulfate concentrations in sedi-ment are generally low compared to coastal marine systems Therefore, freshwatersystems have a greater capacity of retaining phosphorus in the sediment The phos-phorus in freshwater sediment is bound more tightly and proportionally less isreleased back into the water column.35

There is an adsorption–desorption interaction between phosphates and pended particulate matter in the water column The subsequent settling of suspendedsolids together with the adsorbed inorganic phosphorus can be a significant phos-phorus loss mechanism in the water column and is a major source of phosphorus tothe sediment

sus-Although phosphorus exchange by adsorption–desorption within the sedimentand between sediment particles and interstitial water can be as rapid as a few minutes,the rate of phosphorus exchange across the sediment–water interface depends onthe state of the microzone.13

In Thau Lagoon, France, the release of phosphates adsorbed onto Fe(OOH)

low pH.28

4.1.2.5 Significance of N/P Ratio

Net primary production in many marine ecosystems is probably limited by nitrogen,but phosphorus also may limit production in some ecosystems.6–8,34 There is a shiftfrom phosphorus to nitrogen limitation in moving from freshwater to coastal waters.Some of the reasons for this are more efficient recycling of phosphorus,36 the highlosses of nitrogen to the atmosphere due to denitrification in coastal waters,34,36 therole of sulfate in recycling phosphorus in coastal sediment,36 and the low N:P ratio

in nutrient inputs to many coastal waters with limited planktonic nitrogen fixation.34

Differences in nutrient limitation are the result of changes in the ratio of totalnitrogen to total phosphorus in nutrient inputs and the dynamics of internal bio-geochemical processes.34 Analyses of algal cells show that the mean ratio of carbon

to nitrogen to phosphorus is C:N:P=106:16:1 It seems reasonable, therefore, toassume that the cells require these elements according to this ratio Liebig’s law ofminimum states that if the ratio of elements in the water deviates widely from thisratio, elements present in excess cannot be utilized.41 The observed N:P ratios that

are less than 16:1 have been used to indicate that nitrogen is less abundant thanphosphorus with respect to algal (usually phytoplankton) metabolic demand (Red-field Ratio) Boynton et al.42 report that 22 of 27 estuaries surveyed were nitrogenlimited with N:P ratios in the water column well below 16:1 during the time of peakalgal growth In the systems having low N:P ratios, blue-green algae may have acompetitive advantage over other phytoplankton groups due to their ability to fixatmospheric nitrogen.43

There are, however, some problems with using the N:P ratio as an indicator ofnutrient limitation First, different types of algae have different N:P ratios rangingfrom 10:1 to 30:1.41,42 Second, nutrient limitation is often assumed without testing,and other factors may be limiting Third, the use of the water column N:P ratio isbased on the assumption that nutrient loading is constant or at steady state; however,nutrients are often supplied in pulses and the N:P ratio is constantly altered, depend-ing on both the pulse and uptake rates Thus, nutrient ratios in the water column areinsufficient for determining the limiting nutrient for algal growth, especially whenmore than one algal group is present.44 Fourth, the limiting nutrient can changetemporally in a system For example, during winter, when algal crops are sparse andgrowth is slow, the amount of phosphorus present may be sufficient to be nonlimiting

As the growth of algae proceeds during spring, phosphorus is removed from thewater by the algae and the supply becomes progressively depleted.41 Fifth, in thecase of phosphorus, the past history of cells must be considered Many algae seem

to be able to store phosphorus in excess of their present requirements For this reason,sometimes algae may grow at dissolved phosphate concentrations that seem to belimiting The problem is complicated further by the fact that phytoplanktons arefree-moving plants, and thus, they may not have grown in and derived their nutrientsfrom the water in which they were found (spatio-temporal organization) The timedimension is another problem, as discussed in Chapter 2 Nutrients are recycledrapidly through mineralization, and without additional input of phosphorus fromexternal sources the recycled nutrients become available for growth.41

Silicon is present in coastal waters in three principal forms: detrital quartz,aluminosilicate clays, and dissolved silicon Similar to phosphorus, silicon occursprimarily in one oxidation state (+4).27 At the pH and ionic strength of seawater, thedominant dissolved species of silicon is silicic acid (H4SiO4).12

The dominant input of dissolved silicate to most aquatic systems occurs asriverine inputs, as a consequence of weathering reactions in the watershed In order

to become available for biological activity, silicate rocks must be broken down.46–48

Weathering is achieved by a combination of mechanical (physical processes of wind

or ice) and chemical processes (reactions with acidic and oxidizing substances) Therate of chemical weathering varies with the physical conditions of temperature andrainfall amount, and the mineral composition of the rocks Rainwater, springs, andthe leachate from soils are all high in carbon dioxide (carbonic acid) that weathersrocks to release soluble silica.23

The ratios of the nutrients present and the availability of dissolved silicate (DSi)can regulate the species composition of phytoplankton assemblages When concen-trations of DSi become low, other types of algae that do not require DSi can dominatealgal community composition and decrease the relative importance of diatoms inphytoplankton communities.47 There is a considerable concern that altered nutrientratios in coastal waters may favor blooms of nuisance flagellate species that couldreplace the normal spring and autumn bloom of siliceous diatoms.49,50 Large-scalehydrologic alterations on land, such as river damming and river diversion, can causereductions in silicate inputs to the sea.49 This has already been observed in the Blackand Baltic Seas Changes in the nutrient composition of river discharges seem to beresponsible for dramatic shifts in phytoplankton species composition in the BlackSea.51,52 In the Baltic Sea, DSi concentrations and the DSi:N ratio have been decreas-ing since the end of the 1960s, and there are indications that the proportion ofdiatoms in the spring bloom has decreased while flagellates have increased Changingphytoplankton species composition can have repercussions on the entire food web,

Diatom phytoplankton populations are the usual food for zooplankton and feeding fishes, and contribute directly to the large fishable populations in coastalzones Diatoms grow very rapidly, have short lifetimes, are grazed heavily and arerarely a nuisance.46 They tend to dominate ecosystems whenever silicate is abundant

primary production.51 The diatom need for silicate is for the construction of theircell walls (known as frustules) This contrasts with other algae that construct theircell walls from organic material or from calcium carbonate

Silicon does not have a gaseous phase, and its cycle is relatively simple because

it involves only inorganic forms In contrast to other nutrients, particulate andsedimentary silicon decay directly to dissolved inorganic silicon rather than passingthrough a dissolved organic phase (Figure 4.4)

4.1.3.1 Uptake of Silicon

In the silicon cycle, organisms utilize dissolved silicon (H4SiO4) to produce theirskeletons, and this skeletal material dissolves following the death of the organ-isms.25 The half-saturation constant for growth of several diatom species is about0.5–5.0 µM.46 Maximum in situ growth rates between 2–4 d−1 have been repeatedlymeasured for diatoms, whereas the observed maximum growth rates for dinoflagel-late, microflagellate, and eukaryotic nonmotile ultraplankton species or assem-blages have generally been below 2.5 d−1.53

Diatoms use the dissolved silicon together with nitrogen and phosphorus in anaverage Si:N:P ratio of 16:16:1.27,46,54

4.1.3.2 Settling of Diatoms

The mineral content of diatoms increases their specific gravity, which causes negativebuoyancy Diatoms, therefore, tend to sink.7 Increased diatom production results inincreased deposition of silica into sediment Preservation of deposited material insediment depends upon a variety of factors, including pH, salinity, temperature, type

of sediment, and bulk sedimentation rate In general, accumulation of biogenic silica

in sediment mimics overlying water column productivity.47

4.1.3.3 Dissolution of Silica

Silicon is released by dissolution of diatom tests (skeletons) rather than by bially mediated decomposition It has been suggested by Officer and Ryther 46 andConley et al.47 that dissolution rates for particulate silicon are slow relative toregeneration of both nitrogen and phosphorus Regeneration of biogenic silica isprimarily a chemical phenomenon, whereas grazers and bacteria biologically medi-ate the regeneration of nitrogen and phosphorus Both nitrogen and phosphoruswill be recycled faster and reused on shorter time scales than Si This could meanthat midsummer shifts in algal species domination from diatoms to flagellatesmight result from silicon limitation due to relatively slow regeneration However,Day et al.27 report that measurements of benthic nutrient regeneration indicate thatsummer rates of silicon recycling are comparable to those for nitrogen Nixon

micro-et al.55 measured silica fluxes over 1 mmol m−2 h−1 from the sediment duringsummer in Narragansett Bay, U.S.A This flux was higher than commonly supposed.Salinity is a factor that directly affects the dissolution of siliceous minerals Therate of dissolution of biogenic silica increases by a factor of 2 by changing thesalinity from 1 to 5% Thus, dissolution of siliceous minerals is more rapid in marinewaters.47

FIGURE 4.4 Silicon cycle.

Particulate

Particulate Mineral Si uptake

Detrital Quartz Silicon dissolution

dissolution

SEDIMENT WATER

dissolution

An observed release of Si from sediment in Thau Lagoon, France, was attributedmainly to the dissolution of silica that was directly controlled by temperature Incontrast to NH4+ and soluble reactive phosphorus, anaerobic conditions are notsupposed to enhance benthic Si fluxes but rather should decrease the release of Si

by cessation of bioturbation However, this behavior of Si, based on experimentalresults, was not confirmed by the observations in the Thau Lagoon.28

4.1.4 D ISSOLVED O XYGEN

Dissolved oxygen (DO) is considered to be a very important and sensitive indicator

of the health of aquatic systems DO is necessary to support the life functions ofhigher organisms and to drive many redox reactions.56 DO also provides key infor-mation about the system state,57,58 e.g., insight into algal blooms, oxygen depletionrates, and zones of oxygen depletion Changes in the shape of the DO depth curve,

as well as the oxygen deficiency in bottom waters, are meaningful eutrophicationindices.59

Low dissolved oxygen can cause the loss of aquatic animals Most estuarinepopulations can tolerate short exposure to low dissolved oxygen concentrationswithout adverse effects Extended exposures to dissolved oxygen concentrations lessthan 60% oxygen saturation can result in modified behavior, reduced abundance andproductivity, adverse reproductive effects, and mortality.60 For example, ThauLagoon, France, has lost part of its shellfish production over the past decades due

to mass mortality caused by the diffusion of hydrogen sulfide into the water columnduring anoxic periods.28,38

Many aquatic animals have adapted to a short period of hypoxia (dissolvedoxygen concentrations below 3 mg l−1) by taking up more oxygen and transporting

it more effectively to their cells and mitochondria, that is, by ventilating theirrespiratory surfaces more intensely and increasing their heart rate If these responsesare insufficient to maintain the blood’s pH, then the oxygen-carrying capacity willdecrease.61 An early behavioral response is the locomotory, i.e., the moving oforganisms toward better-oxygenated waters even when other conditions there may

swimming and feeding, which will reduce its need for energy and hence oxygen.Although reduced activity may make the animal more hypoxia tolerant for a shortperiod, reduced swimming activity makes the animal more vulnerable to predators,and reduced feeding decreases its growth If oxygen insufficiency persists, deathswill ultimately occur.61

Dissolved oxygen criteria developed by the U.S EPA61 for coastal waters apply

to both continuous and cyclic DO conditions If the DO conditions are always abovethe chronic criterion for growth (4.8 mg l−1), most of the animals and plants cangrow and reproduce unimpaired DO conditions below the juvenile/adult survivalcriterion (2.3 mg l−1) mean that there is not enough DO to protect aquatic life When

DO conditions are persistent between these two values, living organisms oftenbecome stressed and require further evaluation of duration and intensity of low DOevents to determine whether the available levels of oxygen can support a healthyaquatic community.61,63

4.1.4.1 Processes Affecting the Dissolved Oxygen

Balance in Water

The processes affecting the DO balance in aquatic ecosystems include atmosphericreaeration, photosynthesis and respiration, oxidation of organic matter, oxidation ofinorganic matter, sediment oxygen demand, and nitrification.56,64,65 These processesare shown in Figure 4.5

It decreases with increasing temperature and salinity and decreasing partial pressure

of oxygen.45 There are several empirical equations to express the effects of these factors

on the oxygen saturation concentration The following equation can be used to establishthe dependence of oxygen saturation concentration on temperature.68

11 4

Dissolved Oxygen

Surface Exchange

• Algae

• Macrophyte

Photosynthesis Respiration

• Fe 2+

• S 2 −

• NH4+Oxidation

• Organic Matter

• SOD Decomposition

• Fish

where o sf is the saturation concentration of dissolved oxygen in fresh water at 1 atm[mg l−1] and T a is absolute temperature [K].

The following equation can be used to establish the dependence of DO saturationconcentration on salinity.68

(4.8)

where

k a = reaeration rate coefficient [d− 1]

U o = mean current velocity [m s− 1]

U w = wind speed measured 10 m above the water surface [m s− 1]

H = depth [m]

The reaeration rate coefficient can also be affected by temperature and salinity.56

The temperature dependence is often expressed by an Arrhenius type equation.69

Stratification of the water column can prevent diffusion of atmospheric oxygeninto bottom waters, and thus oxygen depletion may occur. 28,70–72 Hypoxic conditions,

in bottom waters can cause mass mortality of benthic organisms and decline in fisheryyields.72 Similar conditions can also occur in semi-enclosed basins with limited waterexchange, such as lagoons.15

4.1.4.1.2 Photosynthesis – Respiration

Photosynthesis is the conversion of simple inorganic nutrients into more complexorganic molecules by autotrophic organisms Via this reaction oxygen is liberatedand CO2 is consumed.45,64

106CO2 + 16NH4 ++ HPO4 −+ 108H2O→ C106H263O110N16P1 + 107O2 + 14H+ (4.9)

The rate of oxygen production is proportional to the growth rate of the phytoplanktonbecause the stoichiometry is fixed An additional source of oxygen is reduction of

NO3− instead of NH4+ for growth When available ammonia nitrogen is exhaustedand the nutrient source is nitrate, nitrate is initially reduced to ammonia, whichproduces oxygen

Respiration is the reverse process of photosynthesis through which oxygen isdiminished in the water column as a result of algal respiration It is normally a smallloss rate for the organism and is temperature dependent.73

Photosynthesis depends on solar radiation; therefore, the production of dissolvedoxygen proceeds only during daylight hours Concurrently with oxygen productionvia photosynthesis, aquatic plants require dissolved oxygen for respiration, whichcan be considered to proceed continuously The photosynthesis and respirationprocesses can add and deplete significant quantities of dissolved oxygen, and thecombined effect of these processes can cause diurnal and seasonal variations indissolved oxygen concentrations in aquatic ecosystems.45,64,66,67 From a seasonalperspective, photosynthesis will tend to dominate during the growing season, whilerespiration and decomposition will prevail during the nongrowing period of aquaticplants Diurnal variations in dissolved oxygen can be induced by light, thus dissolvedoxygen could be supersaturated during the afternoon and depleted severely justbefore dawn during the growing season.45

Heavy growth of rooted and attached macrophytes can cause oxygen uration in water Incidental to this supersaturation, a very high pH value can occurthat is fatal to fish.62 Rooted and attached macrophytes tend to have a greater impact

supersat-on dissolved oxygen than phytoplanktsupersat-ons due to two factors.45

1 Since macrophytes are usually found in shallower water, for an equalgrowth or respiration rate as phytoplankton, the impact of rooted andattached macrophytes on a shallower system will be greater

2 Because they are fixed in space, macrophytes tend to be more concentratedlongitudinally

4.1.4.1.3 Oxidation of Organic Matter

The primary loss mechanism associated with organic matter is oxidation (seeSection 4.2 for details) Organic matter serves as an energy source for heterotrophicorganisms in aerobic respiration and decomposition processes These processesreturn organic matter to the simpler inorganic state During breakdown, oxygen isconsumed and CO2 is liberated.45 A principal source of organic matter, other thananthropogenic pollution and natural run-off, is detritus, produced as a result ofdeath of organisms The biodegradability of organic matter is a key factor affectingthe oxidation rate Thus, different types of organic matter; such as labile dissolvedorganic matter (labile DOM), labile particulate organic matter (labile POM), refrac-tory dissolved organic matter (refractory DOM), and refractory particulate organicmatter (refractory POM) are specified and used in models Labile organic matterdecomposes on a time scale of days to weeks, whereas refractory organic matterrequires more time (e.g., 1 year).45,56 Different oxidation rates are used for each ofthese forms of organic matter in water quality models

Because the oxidation of organic matter is a bacterially mediated process, dation rate is a function of water temperature Temperature correction of the ratecan be made using an Arrhenius type of equation Temperature correction coefficients(θ) range from 1.02 to 1.09 A commonly used value for this coefficient is 1.047.45

oxi-4.1.4.1.4 Oxidation of Inorganic Matter

Under aerobic conditions, most common metals and some nonmetallic elements,like sulfur, are thermodynamically stable in their highest oxidation states Ferric iron(Fe3 +) is more stable than ferrous iron (Fe2 +), and similarly, sulfate is more stablethan sulfide A series of chemical reductions occurs when the oxygen is consumed.All of these reactions tend to reverse if oxygen is reintroduced.23

4.1.4.1.4.1 Sulfide Oxidation

Sulfide is produced in anaerobic sediment as a result of organic matter oxidation,

in which sulfate is used as the electron acceptor A portion of the sulfide precipitates

as FeS(S), while the remaining dissolved sulfide diffuses into the aerobic zone,where it is oxidized back to sulfate Dissolved oxygen is consumed during thislast step.27,74

The particulate sulfide (FeS(S)) can also be mixed into the aerobic zone, where it can

be oxidized by oxygen to Fe2O3(S) A portion of the FeS(S) is buried by sedimentation.74

4.1.4.1.4.2 Iron Oxidation

Fe2 + compounds are more soluble than Fe3 + compounds, and thus exist in the low

mg l−1 range in sediment pore waters As a consequence, Fe2 + can diffuse to the oxiclayer of the sediment, and via the loss of one electron, can be oxidized to Fe3 + bythe oxygen present there.74

1 2

1 4 3 2

Iron oxyhydroxide can react with water.74

4.1.4.1.4.3 Manganese Oxidation

Mn2 + compounds are more soluble than Mn4 + compounds, and therefore exist in the

mg l−1 range in sediment pore waters As a consequence, Mn2 + can diffuse to theoxic layer of the sediment, and via the loss of two electrons, can be oxidized to

Mn4 + by the oxygen present there.74

This reaction occurs under aerobic conditions.74

4.1.4.1.5 Sediment Oxygen Demand

The decomposition of organic material and bioturbation activity of the macrofaunaresult in the exertion of an oxygen demand at the sediment–water interface.38 Thisprocess can have profound effects on the dissolved oxygen concentration in theoverlying waters Oxygen diffuses from the overlying water into the sediment;therefore, organic matter decomposes aerobically in the upper sediment layers thatare in direct contact with water.64 The aerobic surface layer usually has a thickness

of only a few millimeters This is true even for sediment overlaid by oxygen-richwater because of the slow diffusion rates of oxygen.19,27

Oxygen uptake is regulated by the net deposition and degradability of organicmatter.19 In eutrophic systems, phytoplankton settling can have a significant effect onSOD levels.64 Microbial benthic mineralization is a major recycling pathway of settledparticles in shallow marine environments Increased organic loading due to sedimen-tation increases oxygen consumption, which in turn leads to dissolved oxygen deple-tion within the sediment.28 This depletion of oxygen causes SOD to cease.64 In ThauLagoon, France, the accumulation of organic matter in quantities that exceed themineralization capacity of the sediment, high temperatures, and the absence of windtrigger anaerobic bottom conditions.28,38

1 2

1 2

SOD can be determined by in situ measurements or by model calibration if direct measurements from the field are not available In situ measurements are usually

conducted using a chamber Oxygen uptake in the chamber is measured continuouslyover a prespecified period of time, providing the needed data to calculate the oxygenconsumption rate as g O2 m−2 d−1 In a modeling analysis, sediment oxygen demand

is typically formulated as a zero-order process.67 SOD is a function of temperature,and the Arrhenius equation can be used for temperature corrections in the 10–30°Crange Temperature correction coefficients (θ) range from 1.040 to 1.130 A typicalvalue of 1.065 is often used Below 10°C, SOD probably decreases more rapidly andapproaches zero for water in the temperature range of 0–5°C.64 Actual temperaturedependency of the sediment oxygen demand was observed in Sacca di Goro Lagoon,Italy These followed a seasonal trend with pronounced peaks in the warmer months.Specifically, SOD was 80 mmol O2 m2 d−1 in March, and increased to 365 mmol O2

m2 d−1 in August at a sampling station close to a discharge.75

4.1.4.1.6 Nitrification

Nitrification may be a significant oxygen-demanding process as 4.57 mg O2 per mg

NH4+ is consumed during the oxidation of ammonia to nitrate.76 Depressed oxygenlevels in water can inhibit nitrification Therefore, at least 1–2 mg l−1 of dissolvedoxygen is needed to promote nitrification Usually in modeling studies, nitrificationrates are multiplied by a factor that shuts down nitrification as the dissolved oxygenconcentration approaches zero

Changing concentrations of oxygen in the water just above the sediment hasbeen shown to alter the penetration depth of oxygen within the sediment, and isbelieved to be a major controlling factor for nitrification and denitrification processes

in sediment Detailed information on nitrification is given in Section 4.1.1.2

in sediment redox potential, with each reaction occurring within a certain redoxpotential range Redox potential differs according to the sediment type and itslocation In the Gulf of Gdansk, Poland, the sediment redox potentials were in therange of –365 – +246 mV within the system As a result of better oxygenation,

higher E h values were obtained in the sandy sediment as compared to the silty-claysediment These patterns in redox potentials were similar to those observed in othercoastal areas like the Mediterranean and North Seas.77

The sediment depth at which oxygen is exhausted and the redox potential goes

to zero has been defined as the redox potential discontinuity (RPD) layer The RPDlayer is visible due to its change in color and indicates the zone of habitability forbenthic infauna The closer this color change appears to the sediment surface, thelesser is the dissolved oxygen that exists in the sediment porewater.27,61 Due toseasonal changes in the metabolic rates of benthic organisms, the boundary betweenthe oxidized and the reduced sediment often occurs at different depths in the sedimentthroughout the year.30

4.1.5 M ODELING OF N UTRIENT C YCLES

Nutrient cycles are presented in water quality models as a variety of mass balanceequations that contain input, output, and reaction terms (kinetic process equa-tions) In this section, mainly kinetic process equations in the EUTRO5 module

of WASP5 (Water Quality Analysis Simulation Program) are presented WASP

is a dynamic compartment model that can be used to analyze a variety of waterquality problems in such diverse water bodies as coastal waters, estuaries, ponds,streams, lakes, rivers, and reservoirs This model was developed by the U.S

countries.67,79–89

Due to the complexity of nutrient cycles in aquatic ecosystems, various modelsconsider different processes and use different simplifications and assumptions.Therefore, the differences between WASP/EUTRO5 and some other water qualitymodels that can be used for coastal lagoons are also evaluated and the kineticparameters used in various models are presented in tables in this section

EUTRO5 is a module of WASP applicable to modeling eutrophication It ulates eight state variables (ammonium, nitrite/nitrate, orthophosphate, phytoplank-ton biomass, carbonaceous biochemical oxygen demand (CBOD), dissolved oxygen,nonliving organic nitrogen, and nonliving organic phosphorus) in the water columnand sediment bed.78

sim-In EUTRO5, phytoplankton kinetics and dissolved oxygen are considered assystems interacting with the nutrient cycles The model characterizes the phytoplank-ton population as a whole by the total biomass of the phytoplankton present Min-imum formulation for nutrient limitation is used, and constant stoichiometry foralgal biomass is assumed The model does not simulate the kinetics of higher trophiclevel organisms such as zooplankton and fishes Settling terms are not included inthe kinetic equations in the EUTRO5 code, but are coded in the transport sections

of WASP Four types of model segments (surface water, subsurface water, upperbenthic layer, and lower benthic layer) are defined in WASP Six flow fields can beused for calculating the exchanges between segments, which are used for solvingthe mass balance equations

4.1.5.1 Modeling Nitrogen Cycle

Four nitrogen variables are modeled in the EUTRO5 module of the WASP model:phytoplankton nitrogen (living), organic nitrogen (nonliving), ammonium, andnitrate Both ammonium and nitrate are available for phytoplankton growth However,

ammonium is the preferred form because of physiological reasons The ammoniumpreference factor used in the model is given in Equation (4.25) During phytoplanktonrespiration and death, a fraction of the phytoplankton biomass is recycled to thenonliving organic nitrogen pool, while the remaining fraction contributes to theammonium pool Although released ammonium is readily available for algalgrowth/nitrification, released organic nitrogen must undergo mineralization or bac-terial decomposition into ammonium before utilization EUTRO5 uses a saturatingrecycle rate, which is directly proportional to the phytoplankton biomass present (seeEquation (4.22) and Equation (4.23)) Saturating recycle permits second-order depen-dency at low phytoplankton concentrations, and permits first-order recycle when the

phytoplankton concentration greatly exceeds the half-saturation constant, K mPC cally, this mechanism slows the recycle rate if the phytoplankton population is smallbut does not permit the rate to increase continuously as the phytoplankton concen-tration increases

Basi-Equations describing the nitrification of ammonium in EUTRO5 module containtemperature and low dissolved oxygen correction terms The latter is a Monod-typefunction, which represents the decline of nitrification as the dissolved oxygen con-centration approaches zero The denitrification equations also have a key term fordissolved oxygen However, this term is designed to reduce the denitrification rate

as a function of decreasing dissolved oxygen concentration above zero However,

in the benthic layer, where anaerobic conditions always exist, denitrification isalways assumed to occur

ammonium and nitrate The atmospheric deposition of nitrate and biota excretion

of ammonia are two nitrogen sources not modeled in WASP Also in AQUATOXduring the decomposition of detritus only inorganic nitrogen is released, while

in EUTRO5 both inorganic and organic nitrogen are formed Furthermore, threetypes of algae (blue-green, green, and diatoms) plus macrophytes are modeled

in AQUATOX, and each of them has a different nitrogen uptake rate For greenalgae and diatoms, the Redfield ratio may be used Blue-green algae can supplynitrogen by fixation, and this is accounted for by smaller nitrogen uptake ratios

in the model Algae can be either phytoplankton or periphyton Phytoplanktonsare subject to sinking and washout, while periphytons are subject to substratelimitation and scour by currents because they are fixed in space Macrophytessupply their nitrogen from sediment; therefore, nutrient limitation of macrophytes

is not considered in the model Macrophyte ammonium excretion to the watercolumn is modeled

Nitrification and denitrification processes described in AQUATOX have ature and dissolved oxygen correction terms similar to those in EUTRO5; however,

temper-a pH correction term is temper-also included in AQUATOX

nitrogen, ammonium, and nitrite–nitrate–for modeling the nitrogen cycle Similar

to the EUTRO5, phytoplankton is considered as a single population biomass Themineralization rate is modeled as a function of the organic nitrogen concentration

In EUTRO5 mineralization rate is modeled as a function of both the organic nitrogenand phytoplankton concentrations In this model, the nitrification process description

includes a Monod-type function to represent the effect of ammonium concentration

on the nitrification rate Similar to EUTRO5, the dissolved oxygen correction term

is also a Monod-type function

CE-QUAL-W258 has five different organic matter state variables To calculate theamount of nitrogen incorporated into organic matter, two stoichiometric coefficients(nitrogen to organic matter and nitrogen to carbonaceous BOD) are used Thus,organic nitrogen is not a state variable in this model Two of the state variables usedfor modeling the nitrogen cycle are ammonium and nitrate Organic matter decayproduces ammonia Decay of labile and refractory dissolved organic matter pluscarbonaceous BOD form the elements of the dissolved organic nitrogen mineralizationprocess, while decay of labile and refractory particulate organic matter corresponds

to hydrolysis plus mineralization Unlike EUTRO5, which simulates phytoplanktons

as a single population biomass, CE-QUAL-W2 can simulate an unlimited number ofalgal and epiphyton groups, each group/member having different half-saturation con-stants and uptake rates for nitrogen For the nitrification and denitrification, CE-QUAL-W2 and EUTRO5 both have a temperature correction term; however, theiroxygen correction functions are different In EUTRO5, the effect of dissolved oxygenconcentration on nitrification and the denitrification rates are expressed as Monod-type functions In CE-QUAL-W2, the nitrification and denitrification terms do notinclude such functions, but rather include discrete sign functions These sign functionsshut down nitrification if the dissolved oxygen concentration is less than a prespecifiedminimum value On the contrary, denitrification shuts down when dissolved oxygenconcentrations exceed the same minimum value In CE-QUAL-W2, nitrogen fixation

by blue-green algae can be modeled by setting the half-saturation concentration fornitrogen to zero for the corresponding algal group(s)

CE-QUAL-R157 also uses ammonium and nitrate as state variables Similar toCE-QUAL-W2, the organic matter is represented by state variables, namely, labiledissolved organic matter, refractory dissolved organic matter, and detritus Aerobicdecomposition of the organic matter contributes to the ammonium pool Respiration

of three types of phytoplankton, one type of macrophyte, zooplankton, and fish arealso included as ammonium sources Nitrification is allowed only in layers withdissolved oxygen concentrations greater than a prespecified minimum value, which

is also the case in CE-QUAL-W2

The CE-QUAL-ICM56 model has five state variables, namely ammonium, nitrate,dissolved organic nitrogen, labile particulate organic nitrogen, and refractory particu-late organic nitrogen Three algae groups are defined: green algae, diatoms, and blue-green algae In CE-QUAL-ICM, the rate of dissolved organic nitrogen mineralization

is related to algal biomass When dissolved inorganic nitrogen (ammonium + nitrate)

is scarce, algae stimulate production of an enzyme that mineralizes dissolved organicnitrogen to ammonium Mineralization rate is highest when algae are strongly nitrogenlimited; it is lowest when no limitation exists Similar calculations are also made forhydrolysis of the labile and refractory particulate organic nitrogen Similar to the case

in the model developed by Park and Kuo,69 the nitrification term in CE-QUAL-ICMincludes a Monod-type function representing the effect of ammonium concentration

on nitrification Similar to EUTRO5, the dissolved oxygen correction term is also aMonod-type function

The HEM-3D90 model uses kinetic processes mostly from the CE-QUAL-ICMmodel Macroalgae are added into HEM-3D as the fourth algal group.

The specific kinetic equations used in the EUTRO5 model for nitrogen (in almostthe same or similar format as for other models) are presented below

4.1.5.1.1 Phytoplankton Nitrogen

Phytoplankton nitrogen is the nitrogen contained in phytoplankton cells

(4.21)

where

C4 = phytoplankton carbon concentration [mg C l− 1]

anc= nitrogen to carbon ratio [mg N/mg−1 C]

G P1= specific growth rate constant of phytoplankton [day−1]

D P1= death plus respiration rate constant of phytoplankton [day−1]

V s4= net settling velocity of phytoplankton [m day−1]

D = depth of segment [m]

4.1.5.1.2 Organic Nitrogen

(4.22)

where

C7 = organic nitrogen concentration [mg N l− 1]

D P1 = death plus respiration rate constant of phytoplankton [day− 1]

anc = nitrogen to carbon ratio [mg N mg− 1 C]

fon= fraction of dead and respired phytoplankton recycled to the organic nitrogen pool [none]

C4= phytoplankton carbon concentration [mg C l−1]

k71= organic nitrogen mineralization rate constant at 20°C [day−1]

θ71 = organic nitrogen mineralization temperature coefficient [none]

T = water temperature [°C]

K mPC = half-saturation constant for recycle [mg C l− 1]

V S3 = organic matter settling velocity [m day− 1]

f D7= fraction of dissolved organic nitrogen [none]

1 – f D7 = fraction of particulate organic nitrogen [none]

D = depth of segment [m]

Literature values, obtained from various coastal areas, for the kinetic rate constantsrelated to organic nitrogen transformations, their temperature correction coefficients,and settling velocities of organic nitrogen are given in Tables 4.2–4.5

D

7 P1 nc on 4 71 71

TABLE 4.2

PON to DON Rate Constant (day−1)

Min Max Avg Location Reference As Cited in Explanation

0.075 Chesapeake

Bay

Cerco and Cole, 1994

Cerco and Cole 91

Labile PON

to DON 0.005 Chesapeake

Bay

Cerco and Cole, 1994

Cerco and Cole 91

Rate Constants for Decomposition of Organic Nitrogen to NH 4+ (day−1 )

Min Max Avg Location Reference As Cited in Explanation

0.14 Chesapeake

Bay

Salas and Thomann, 1978

Bowie et al 93 PON to NH4+

0.03 Chesapeake

Bay

Hydroqual Inc., 1987

Lung and Paerl 79 0.005 0.05 Texas bays and

Estuary

Lung and Paerl, 1986

Lung and Paerl 79 0.08 Manasquan

Estuary

Najarian et al., 1984

Given as minimum mineralization rate of DON 0.075 Maryland

coastal waters

Lung and Hwang, 1994

Lung and Hwang 81

DON to NH4+, at

20 ° C, value used in modeling

Estuary

Bunch et al., 2000

Bunch et al 92 DON to NH4+

4.1.5.1.3 Ammonium Nitrogen

(4.23)

where

C1 = ammonium nitrogen concentration [mg N l− 1]

D P1 = death plus respiration rate constant of phytoplankton [day− 1]

anc = nitrogen to carbon ratio [mg N/mg− 1 C]

1 − fon = fraction of dead and respired phytoplankton recycled to the ammonium

nitrogen pool [none]

C4= phytoplankton carbon concentration [mg C l−1]

k71= organic nitrogen mineralization rate constant at 20°C [day−1]

θ71 = organic nitrogen mineralization temperature coefficient [none]

TABLE 4.4

Temperature Correction Factors for Decomposition

of Organic Nitrogen to NH 4+ (θ)

Average Location Reference As Cited in Explanation

1.03 Texas bays and

DON to NH4+, value used

in modeling

TABLE 4.5 Settling Velocity of Organic Nitrogen (m day−1 )

Min Max Avg Location Reference As Cited in

0.23 James River O’Connor, 1981 O’Connor 94 0.4 Patuxent River O’Connor, 1981 O’Connor 94 0.15 Patuxent River Estuary Lung, 1992 Lung 80 0.6 Sacramento San Joaquin Delta O’Connor, 1981 O’Connor 94

P

T NIT

4

T = water temperature [°C]

K mPC = half-saturation constant for recycle [mg C l− 1]

C7 = organic nitrogen concentration [mg N l− 1]

G P1 = specific growth rate constant of phytoplankton [day− 1] = preference for ammonium uptake [none]

k12 = nitrification rate constant at 20°C [day− 1]

θ12 = nitrification temperature coefficient [none]

KNIT = half-saturation constant for oxygen limitation of nitrification [mg O2 l−1]

C6 = dissolved oxygen concentration [mg O2 l−1]Table 4.6 and Table 4.7 contain nitrification rate constants and temperaturecorrection factors for nitrification in coastal waters, respectively

4.1.5.1.4 Nitrate Nitrogen

(4.24)where

(4.25)Ammonium preference factor

C2 = nitrate–nitrogen concentration [mg N l− 1]

k12 = nitrification rate constant at 20°C [day− 1]

θ12 = nitrification temperature coefficient [none]

T = water temperature [°C]

KNIT= half-saturation constant for oxygen limitation of nitrification [mg O2 l−1]

C6 = dissolved oxygen concentration [mg O2 l−1]

C1 = ammonium nitrogen concentration [mg N l− 1]

G P1 = specific growth rate constant of phytoplankton [day− 1]

anc = nitrogen to carbon ratio [mg N mg− 1 C]

C4 = phytoplankton carbon concentration [mg C l− 1]

k2D = denitrification rate constant at 20°C [day−1]

θ2D = denitrification temperature coefficient [none]

KNO3 = half-saturation constant for oxygen limitation of denitrification [mg O2 l−1]

K mN = half-saturation constant for inorganic nitrogen [mg N l− 1]

TABLE 4.6

Table 4.8 and Table 4.9 contain denitrification rate constants and temperaturecorrection factors for denitrification in coastal waters, respectively Nitrogen fixationrate constants used for coastal areas are given in Table 4.10.

The EUTRO5 model also includes benthic equations because the decomposition

of organic material in benthic sediment can have profound effects on the trations of oxygen and nutrients in the overlying waters

concen-In CE-QUAL-W2,58 two processes are defined to simulate ammonium release fromsediment One process is a first-order process that is coupled with the aerobic organicmatter decay in sediment The other is a zero-order process that occurs when the

TABLE 4.7

Nitrification Temperature Correction Factor (θ)

1.045 Potomac River Estuary Thomann and Fitzpatrick, 1982 Bowie et al 93

1.05 Manasquan Estuary Najarian et al., 1984 Najarian et al 95 1.08 Venice Lagoon Melaku Canu et al., 2001 Melaku Canu et al 88 1.08 Patuxent River Estuary Lung, 1992 Lung 80

1.08 Maryland coastal waters Lung and Hwang, 1994 Lung and Hwang 81

TABLE 4.8 Denitrification Rate Constant (day−1 )

Average Location Reference As Cited in Explanation

0.09 Potomac River

Estuary

Thomann and Fitzpatrick, 1982

Denitrification Temperature Correction Factor (θ)

1.045 Potomac River Estuary Thomann and Fitzpatrick, 1982 Bowie et al 93 1.045 New York Bight O’Connor et al., 1981 Bowie et al 93 1.045 Patuxent River Estuary Lung, 1992 Lung 80

1.08 Maryland coastal waters Lung and Hwang, 1994 Lung and Hwang 81

TABLE 4.10

(Baltic)

Sweden

and well-developed cyanobacterial mats

Rhode Island

and well-developed cyanobacterial mats

and well-developed cyanobacterial mats

and well-developed cyanobacterial mats

Beaufort Sea

and well-developed cyanobacterial mats

and well-developed cyanobacterial mats

0.14 Lune Estuary, U.K Jones, 1982 Howarth et al 31 Calculated values for macrophyte-free sediments

and well-developed cyanobacterial mats

Florida

and well-developed cyanobacterial mats

and well-developed cyanobacterial mats

dissolved oxygen concentration is less than a prespecified minimum value at whichanaerobic processes begin The minimum dissolved oxygen default value is 0.1 mg l−1.

macrophytes from sediment is included Anaerobic ammonium release, aerobic position of organic matter, and settling of adsorbed ammonium are also considered inthe sediment compartment model balance equations

sediment submodel developed by Cerco and Cole56 for the Chesapeake Bay ication model study This submodel consists of three basic processes: deposition ofparticulate organic matter to the sediment, organic matter diagenesis (decay), andrelease of nutrients (phosphate, ammonium, and nitrate) to the overlying watercolumn or their burial into deeper sediment

eutroph-Benthic equations for organic, ammonium, and nitrate nitrogen are given in thefollowing subsections

4.1.5.1.5 Organic Nitrogen (Benthic):

(4.26)Algal decomposition Mineralization

where

C7= organic nitrogen concentration [mg N l−1]

kPZD = anaerobic algal decomposition rate constant at 20°C [per day− 1]

θPZD = anaerobic algal decomposition temperature coefficient [none]

T = temperature [°C]

anc = nitrogen to carbon ratio [mg N mg− 1 C]

fon= fraction of dead and respired phytoplankton recycled to the organic nitrogen pool [none]

C4 = phytoplankton carbon concentration [mg C l− 1]

kOND = organic nitrogen decomposition rate constant at 20°C [day− 1]

θOND = organic nitrogen decomposition temperature coefficient [none]

4.1.5.1.6 Ammonia Nitrogen (Benthic)

(4.27)Algal decomposition Mineralization

where

C1= ammonium nitrogen concentration [mg N l−1]

kPZD= anaerobic algal decomposition rate constant at 20°C [day−1]

θPZD = anaerobic algal decomposition temperature coefficient [none]

T = temperature [°C]

anc = nitrogen to carbon ratio [mg N mg− 1 C]

1 − fon= fraction of dead and respired phytoplankton recycled to the ammonia

nitrogen pool [none]

nc on 4 OND OND

( 20) 7

PZD PZD ( )

( )

C4 = phytoplankton carbon concentration [mg C l− 1]

kOND = organic nitrogen decomposition rate constant at 20°C [day− 1]

θOND = organic nitrogen decomposition temperature coefficient [none]Ammonium nitrogen and organic nitrogen transformation rate constants in the sed-iment of coastal waters and related temperature correction factors are given inTables 4.11–4.13 Although EUTRO5 does not include nitrification in sediment, thevalues given in Table 4.11 can be useful for other models

TABLE 4.11

Sediment Nitrification Rate (mg N m−2 day−1 )

20 Kingoodie Bay, U.K Macfarlene and

112 Norsminde Fjord, Denmark Binnerup et al., 1992 Herbert 8

84 Ochlockonee Bay, Florida Seitzinger, 1987 Herbert 8

23 Narragansett Bay, U.S.A Seitzinger

Estuary, U.S.A.

Lung, 1992 Lung 80 Given as org N

decomposition rate, value used

in modeling

Sediment NH4+ release rates for coastal marine environments are given inTable 4.14.

4.1.5.1.7 Nitrate Nitrogen (Benthic)

(4.28)

where

C2 = nitrate nitrogen concentration [mg N l− 1]

k2D= denitrification rate constant at 20°C [day−1]

θ2D = denitrification temperature coefficient [none]

T = water temperature [°C]

Sediment oxidized nitrogen release rates compiled from the literature are given

in Table 4.15 Half-saturation concentrations for nitrate and sediment denitrificationrates are presented in Tables 4.16–4.17

4.1.5.2 Modeling of Phosphorus Cycle

The three phosphorus variables modeled in EUTRO5 are phytoplankton phosphorus(phosphorus incorporated in phytoplankton cells), organic phosphorus, and inorganicphosphorus (orthophosphate) Organic and inorganic phosphorus are divided intotheir particulate and dissolved fractions

Dissolved inorganic phosphorus is incorporated into phytoplankton cells duringgrowth During phytoplankton respiration and death, organic and dissolved inorganicphosphorus are released Upon release, dissolved inorganic phosphorus is readily avail-able for algal growth and the released organic phosphorus must undergo mineralization

or bacterial decomposition before it can be utilized by phytoplankton Similar to organicnitrogen mineralization, the organic phosphorus mineralization term in EUTRO5includes a saturating recycle rate that is directly proportional to the phytoplanktonbiomass present

TABLE 4.13 Temperature Correction Factor for Organic Nitrogen Decomposition in Sediments to NH 4+ (θ)

1.04 Texas bays and estuaries Brandes, 1976 Bowie et al 93 1.18 Patuxent River Estuary Lung, 1992 Lung 80

D D ( ) Denitrification